Method for depositing very thin PECVD SiO2 in 0.5 micron and 0.35 micron technologies

ABSTRACT

A very thin (less than 350 angstrom) layer of silicon dioxide (SiO 2 ) is produced using plasma-enhanced chemical vapor deposition (PECVD) by substantially increasing the time duration of pre-coat and soak time steps of the PECVD process and substantially reducing the flow of silane (SiH 4 ), the applied high frequency power and the applied pressure in the PECVD process.

FIELD OF THE INVENTION

This invention relates to the field of integrated circuit fabricationmethods and, more specifically, to integrated circuit fabricationmethods for depositing a very thin layer of silicon dioxide (SiO₂).

BACKGROUND OF THE INVENTION

Various integrated circuits utilize structures formed by a thin layer ofsilicon dioxide (SiO₂) for various purposes. For example, a thin layerof SiO₂ is used as a protection structure for on-chip resistors. Asintegrated circuit technologies become smaller, it is advantageous forall structures to become smaller, including thin SiO₂ layers.

Suitably thin SiO₂ layers can be formed using a conventional method ofthermal oxide deposition. However, the high thermal budget associatedwith thermal oxide consumes silicon and drives source/drain (S/D)implantation further so that the S/D implant is not easily controlled.

An advantageous alternative to thermal oxide deposition of a thin SiO₂layer is deposition using plasma-enhanced chemical vapor deposition(PECVD) technique. However, conventional PECVD methods do not allowdeposition of layers less than about 1000 angstroms.

What is needed is a fabrication method that repeatably produces a highquality, uniform and very thin PECVD SiO₂ layer.

SUMMARY OF THE INVENTION

In accordance with the present invention, a very thin (less than 350angstrom) layer of silicon dioxide (SiO₂) is produced usingplasma-enhanced chemical vapor deposition (PECVD) by substantiallyincreasing the time duration of pre-coat and soak time steps of thePECVD process and substantially reducing the flow of silane (SiH₄), theapplied high frequency power and the applied pressure in the PECVDprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are specifically setforth in the appended claims. However, the invention itself, both as toits structure and method of operation, may best be understood byreferring to the following description and accompanying drawings.

FIG. 1 is a cross-sectional view of an integrated circuit wafer showingan example of a thin PECVD SiO₂ layer deposited for resistor protection.

FIG. 2 is a pictorial representation of a method for depositing a thinSiO₂ layer in accordance with one embodiment of the present invention.

FIG. 3 is a flow chart which illustrates steps of a method fordepositing a thin SiO₂ layer in accordance with the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a cross-sectional view of an integrated circuitwafer 100 shows an example of a thin PECVD SiO₂ layer 102 for a resistorprotection deposition. The PECVD SiO₂ layer 102 overlies a siliconsubstrate 104. Titanium silicide (TiSi₂) 106 is deposited after thePECVD SiO₂ layer 102 is formed on the silicon substrate 104 byconventional salicide process techniques so that the silicon substrate104 underlying the PECVD SiO₂ layer 102 is blocked and therefore notaffected by the salicide process. In this manner, an in-chip resistor isformed. The PECVD SiO₂ layer 102 produced using a method illustrated inFIGS. 2 and 3 forms a PECVD SiO₂ layer 102 of about 350 angstroms orsmaller.

Referring to FIG. 2, a PECVD reactor 200 for depositing a thin SiO₂layer of 350 angstroms or smaller is shown. In one embodiment of themethod, a NOVELLUS CONCEPT 1 (TM) PECVD reactor 200 is used to depositSiO₂ in a silane (SiH₄)-based system. For processing 8" (approximately200 mm) semiconductor wafers, a 200 mm system having 5 shower heads forapplying reactant gases deposits the 350 angstrom SiO₂ layer so thatabout 70 angstroms per shower head are applied. For processing 6"(approximately 150 mm) semiconductor wafers, a 150 mm system having 7shower heads for applying reactant gases deposits the 350 angstrom SiO₂layer so that about 50 angstroms per shower head are applied. In theseembodiments, reactant gases include silane (SiH₄), nitrous oxide (N₂ O)and molecular nitrogen (N₂).

The reactor 200 includes a chamber 202 holding a wafer 204, an in-flowtube 206 for carrying reactant gases to the chamber 202, a shower head208 for applying the reactant gases to the chamber 202. A heater block210 heats the wafer 204 and also supports the wafer 204 duringprocessing.

To deposit thin SiO₂ in accordance with an embodiment of the presentinvention the flow of silane SiH₄ is substantially reduced, the RF powerapplied to the reactor 200 is substantially reduced and the pressureapplied to the reactor 200 is substantially reduced.

Referring to FIG. 3, a flow chart illustrates steps of a method 300 fordepositing a thin SiO₂ layer. A precoat time, as is known in the PECVDart, is employed for chamber seasoning in precoat step 302. During aprecoat operation, the wafer 204 is held outside the reactor 200 whilereactant gases are applied to the chamber 202. The precoat step 302coats the interior surfaces of the reactor 200. In precoat step 302, aprecoat time that is substantially longer than a precoat time for aconventional PECVD technique is utilized. For example, a standardprecoat time using the NOVELLUS CONCEPT 1 (TM) PECVD reactor isapproximately 60 seconds. The precoat time for an embodiment of theinventive method is substantially increased to about 300 seconds,although increases to 285 seconds or more are suitable. During, theincreased-duration precoat step 302, the RF plasma is activatedsufficiently long for deposition repeatability and production of adenser film.

A wafer temperature soak operation, as is known in the PECVD art, isapplied in soak step 304. During the soak operation, a cold wafer 204from outside the reactor 200 is placed into the chamber 202 and heatedon the heaterblock 210 before reactant gases, power and pressure areapplied to the chamber 202. In a soak step 302, a soak time that issubstantially longer than a soak time for a conventional PECVD techniqueis utilized. For example, a standard soak time using the NOVELLUSCONCEPT 1 (TM) PECVD reactor is approximately 9 seconds. The soak timefor an embodiment of the inventive method is substantially increased toabout 18.5 seconds, although increases to 17 seconds or more aresuitable.

The wafer temperature soak time and precoat time are increased toprevent the thin PECVD SiO₂ layer from showing a haze appearance and aporous condition. Such an appearance and condition are indicative ofvery poor thin film properties.

In deposition step 306, the reactants including silane (Si₄), nitrousoxide (N₂ O) and molecular nitrogen (N₂) are applied by a flow into thechamber 202 at a selected pressure and RF power. In a deposition step306, deposition parameters that are substantially different from thedeposition parameters for a conventional PECVD technique are utilized.

For example, standard deposition parameters using the NOVELLUS CONCEPT 1(TM) PECVD reactor to fabricate 6" (150 mm) wafers include reactant gasflow rates of 200 sccm for silane (SiH₄), 6000 sccm for nitrous oxide(N₂ O) and 3150 sccm for nitrogen (N₂). Conventionally, RF power isapplied at 1000 watts at a pressure of 1.5 torr. The depositiontemperature is 400 degrees Celsius. Using these deposition parameters, adeposition rate of 5500 angstroms per minute is achieved.

In an embodiment of the present invention using the NOVELLUS CONCEPT 1(TM) PECVD reactor to fabricate 6" (150 mm) wafers, the silane (SiH₄)flow rate is reduced to about 70 sccm, although flow rates from 65 sccmto 75 sccm are suitable. The nitrous oxide (N₂ O) flow rate is reducedto about 4000 sccm, although flow rates from 3900 sccm to 4100 sccm aresuitable. Nitrogen flow rates from 3100 sccm to 3300 sccm are suitableand do not differ from the nitrogen flow rates of the conventionalprocess. RF power is reduced to about 500 watts although power in arange from 480 watts to 520 watts is suitable. Pressure is applied atabout 1.5 torr although pressures from 1.4 torr to 1.6 torr aresuitable. The deposition temperature is 400 degrees Celsius andsubstantially unchanged from the conventional process. Using thesedeposition parameters, a deposition rate is lowered to about 1700angstroms per minute. Deposition is applied for about 2.4 seconds.

In another example, standard deposition parameters using the NOVELLUSCONCEPT 1 (TM) PECVD reactor to fabricate 8"(200 mm) wafers includereactant gas flow rates of 300 sccm for silane (SiH₄), 9500 sccm fornitrous oxide (N₂ O) and 1500 sccm for nitrogen (N₂). Conventionally, RFpower is applied at 1100 watts at a pressure of 2.4 torr. The depositiontemperature is 400 degrees Celsius. Using these deposition parameters, adeposition rate of 5400 angstroms per minute is achieved.

In an embodiment of the present invention using the NOVELLUS CONCEPT 1(TM) PECVD reactor to fabricate 8" (200 mm) wafers, the silane (SiH₄)flow rate is reduced to about 100 sccm, although flow rates from 95 sccmto 105 sccm are suitable. The nitrous oxide (N₂ O) flow rate is reducedto about 6500 sccm, although flow rates from 6400 sccm to 6600 sccm aresuitable. Nitrogen flow rates from 2000 sccm to 2200 sccm are suitableand do not differ from the nitrogen flow rates of the conventionalprocess. RF power is reduced to about 500 watts although power in arange from 480 watts to 520 watts is suitable. Pressure is applied atabout 1.6 torr although pressures from 1.5 torr to 1.7 torr aresuitable. The deposition temperature is 400 degrees Celsius andsubstantially unchanged from the conventional process. Using thesedeposition parameters, a deposition rate is lowered to about 1700angstroms per minute. Deposition is applied for about 2.4 seconds.

Thin film properties are substantially improved using the disclosedmethod with the SiO₂ improving in silicon richness as is shown by arefractor 4 index (RI) that improves from 1.465 using the conventionalmethod to an RI of 1.476 for 8" wafers and to an RI of 1.480 for 6"wafers. Similarly, the disclosed process improves density of the SiO₂film as is shown by an improvement in wet etch rate from 338 angstromsper minute using the conventional process to a wet etch rate of 245angstroms per minute for 8" wafers and to a wet etch rate of 242angstroms per minute for 6" wafers. Similarly, uniformity issubstantially improved using the disclosed method. Same-wafervariability in thickness of 2.37 angstroms and a wafer to wafervariability of 2.58 angstroms are measured using the conventionalprocess. In the disclosed method for 8" wafers, same-wafer variabilityin thickness is improved to 0.45 angstroms and a wafer to wafervariability is improved to 0.8 angstroms. In the disclosed method for 6"wafers, same-wafer variability in thickness is improved to 0.51angstroms and a wafer to wafer variability is improved to 1.0 angstroms.

The description of certain embodiments of this invention is intended tobe illustrative and not limiting. Numerous other embodiments will beapparent to those skilled in the art, all of which are included withinthe broad scope of this invention. For example, A thin layer of siliconis grown by PECVD SiO₂ deposition using Silane (SiH₄) as the siliconsource. In other embodiments of the fabrication method, other sources ofsilicon may be employed, including silicon tetrachloride (SICl₄),trichlorosilane (SiHCl₃) or dichlorosilane (SiH₂ Cl₂). Silane anddichorosilane are typically used for depositing relatively thin siliconepitaxial layers and for depositing epitaxial layers at a relatively lowtemperature.

What is claimed is:
 1. A method of depositing a thin PECVD SiO₂ layerusing a PECVD reactor comprising the steps of:precoating the PECVDreactor for a minimum time duration of 285 seconds; wafer temperaturesoaking a semiconductor wafer for a minimum time duration of 17 seconds;and applying reactant gases to the wafer under pressure in a range from1.4 torr to 1.7 torr and RF power in a range from 480 watts to 520watts, the reactant gases including:silane (SiH₄) at a flow rate in arange from 65 sccm to 105 sccm; nitrous oxide (N₂ O) at a flow rate in arange from 3900 sccm to 6600 sccm, and nitrogen (N₂) at a flow rate in arange from 2000 sccm to 3300 sccm.
 2. A method according to claim 1wherein: the wafer is a 6" wafer;reactant gases are applied to the waferunder pressure in a range from 1.4 torr to 1.6 torr and RF power in arange from 480 watts to 520 watts, the reactant gases including:silane(SiH₄) at a flow rate in a range from 65 sccm to 75 sccm; nitrous oxide(N₂ O) at a flow rate in a range from 3900 sccm to 4100 sccm, andnitrogen (N₂) at a flow rate in a range from 3100 sccm to 3300 sccm. 3.A method according to claim 2 wherein:reactant gases are applied to thewafer under pressure of approximately 1.5 torr and RF power ofapproximately 500 watts, the reactant gases including:silane (SiH₄) at aflow rate of approximately 70 sccm; nitrous oxide (N₂ O) at a flow rateof approximately 4000 sccm; and nitrogen (N₂) at a flow rate ofapproximately 3200 sccm.
 4. A method according to claim 1 wherein:thewafer is a 8" wafer; reactant gases are applied to the wafer underpressure in a range from 1.6 torr to 1.7 torr and RF power in a rangefrom 480 watts to 520 watts, the reactant gases including:silane (SiH₄)at a flow rate in a range from 95 sccm to 105 sccm; nitrous oxide (N₂ O)at a flow rate in a range from 6400 sccm to 6600 sccm, and nitrogen (N₂)at a flow rate in a range from 2000 sccm to 2200 sccm.
 5. A methodaccording to claim 4 wherein:reactant gases are applied to the waferunder pressure of approximately 1.6 torr and RF power of approximately500 watts, the reactant gases including:silane (SiH₄) at a flow rate ofapproximately 100 sccm; nitrous oxide (N₂ O) at a flow rate ofapproximately 6500 sccm; and nitrogen (N₂) at a flow rate ofapproximately 2100 sccm.
 6. A method of depositing a thin PECVD SiO₂layer using a PECVD reactor comprising the steps of:precoating the PECVDreactor for a time duration greater than or equal to 285 seconds; wafertemperature soaking a semiconductor wafer for a time duration greaterthan or equal to 17 seconds; and applying reactant gases to thesemiconductor wafer under pressure in a range from 1.4 torr to 1.7 torrand RF power in a range from 480 watts to 520 watts, the reactant gasesincluding:a silicon source reactant gas at a flow rate in a range from65 sccm to 105 sccm; nitrous oxide (N₂ O) at a flow rate in a range from3900 sccm to 6600 sccm, and nitrogen (N₂) at a flow rate in a range from2000 sccm to 3300 sccm.
 7. A method according to claim 6 wherein thesilicon source reactant gas is selected from silicon tetrachloride(SiCl₄), trichlorosilane (SiHCl₃), and dichlorosilane (SiH₂ Cl₂).
 8. Amethod according to claim 6 wherein:the wafer is a 6" wafer; reactantgases are applied to the wafer under pressure in a range from 1.4 torrto 1.6 torr and RF power in a range from 480 watts to 520 watts, thereactant gases including:the silicon source reactant gas at a flow ratein a range from 65 sccm to 75 sccm; nitrous oxide (N₂ O) at a flow ratein a range from 3900 sccm to 4100 sccm, and nitrogen (N₂) at a flow ratein a range from 3100 sccm to 3300 sccm.
 9. A method according to claim 8wherein the silicon source reactant gas is selected from silicontetrachloride (SiCl₄), trichlorosilane (SiHl₃), and dichlorosilane (SiH₂Cl₂).
 10. A method according to claim 9 wherein:reactant gases areapplied to the wafer under pressure of approximately 1.5 torr and RFpower of approximately 500 watts, the reactant gases including:thesilicon source reactant gas at a flow rate of approximately 70 sccm;nitrous oxide (N₂ O) at a flow rate of approximately 4000 sccm; andnitrogen (N₂) at a flow rate of approximately 3200 sccm.
 11. A methodaccording to claim 6 wherein:the wafer is a 8" wafer; reactant gases areapplied to the wafer under pressure in a range from 1.6 torr to 1.7 torrand RF power in a range from 480 watts to 520 watts, the reactant gasesincluding:the silicon source reactant gas at a flow rate in a range from95 sccm to 105 sccm; nitrous oxide (N₂ O) at a flow rate in a range from6400 sccm to 6600 sccm, and nitrogen (N₂) at a flow rate in a range from2000 sccm to 2200 sccm.
 12. A method according to claim 11 wherein thesilicon source reactant gas is selected from silicon tetrachloride(SiCl₄), trichlorosilane (SiHCl₃). and dichlorosilane (SiH₂ Cl₂).
 13. Amethod according to claim 12 wherein:reactant gases are applied to thewafer under pressure of approximately 1.6 torr and RF power ofapproximately 500 watts, the reactant gases including:the silicon sourcereactant gas at a flow rate of approximately 100 sccm; nitrous oxide (N₂O) at a flow rate of approximately 6500 sccm; and nitrogen (N₂) at aflow rate of approximately 2100 sccm.
 14. A method of fabricating anintegrated circuit comprising the steps of:depositing a thin PECVD SiO₂layer using a PECVD reactor; precoating the PECVD reactor for a timeduration of 285 or more seconds; wafer temperature soaking asemiconductor wafer for a time duration of 17 or more seconds; andapplying reactant gases to the semiconductor wafer under pressure in arange from 1.4 torr to 1.7 torr and RF power in a range from 480 wattsto 520 watts, the reactant gases including:a silicon source reactant gasat a flow rate in a range from 65 sccm to 105 sccm; nitrous oxide (N₂ O)at a flow rate in a range from 3900 sccm to 6600 sccm, and nitrogen (N₂)at a flow rate in a range from 2000 sccm to 3300 sccm.
 15. A methodaccording to claim 14 wherein the silicon source reactant gas isselected from silicon tetrachloride (SiCl₄), trichlorosilane (SiHCl₃),and dichlorosilane (SiH₂ Cl₂).
 16. A method according to claim 14wherein:the wafer is a 6" wafer; reactant gases are applied to the waferunder pressure in a range from 1.4 torr to 1.6 torr and RF power in arange from 480 watts to 520 watts, the reactant gases including:thesilicon source reactant gas at a flow rate in a range from 65 sccm to 75sccm; nitrous oxide (N₂ O) at a flow rate in a range from 3900 sccm to4100 sccm, and nitrogen (N₂) at a flow rate in a range from 3100 sccm to3300 sccm.
 17. A method according to claim 16 wherein the silicon sourcereactant gas is selected from silicon tetrachloride (SiCl₄),trichlorosilane (SiHCl₃), and dichlorosilane (SiH₂ Cl₂).
 18. A methodaccording to claim 17 wherein:reactant gases are applied to the waferunder pressure of approximately 1.5 torr and RF power of approximately500 watts, the reactant gases including:the silicon source reactant gasat a flow rate of approximately 70 sccm; nitrous oxide (N₂ O) at a flowrate of approximately 4000 sccm; and nitrogen (N₂) at a flow rate ofapproximately 3200 sccm.
 19. A method according to claim 14 wherein:thewafer is a 8" wafer; reactant gases are applied to the wafer underpressure in a range from 1.6 torr to 1.7 torr and RF power in a rangefrom 480 watts to 520 watts, the reactant gases including:the siliconsource reactant gas at a flow rate in a range from 95 sccm to 105 sccm;nitrous oxide (N₂ O) at a flow rate in a range from 6400 sccm to 6600sccm, and nitrogen (N₂) at a flow rate in a range from 2000 sccm to 2200sccm.
 20. A method according to claim 19 wherein the silicon sourcereactant gas is selected from silicon tetrachloride (SICl₄),trichlorosilane (SiHCl₃), and dichlorosilane (SiH₂ Cl₂).
 21. A methodaccording to claim 20 wherein:reactant gases are applied to the waferunder pressure of approximately 1.6 torr and RF power of approximately500 watts, the reactant gases including:the silicon source reactant gasat a flow rate of approximately 100 sccm; nitrous oxide (N₂ O) at a flowrate of approximately 6500 sccm; and nitrogen (N₂) at a flow rate ofapproximately 2100 sccm.